Normally the body protects its muscle protein from excessive breakdown. In CF, however, the protein is broken down at an abnormally high rate. This process is worse during illness but occurs even when patients are in thebr usual, stable state of health. It is clinically relevant because lung muscles are affected, and this may impair pulmonary function. The majority of adults with CF are insulin deficient, meaning that their pancreas makes less than normal amunts of insulin in response to blood sugar. Insulin is a hormone which helps the body to sonserve muscle protein. We believe that protein breakdown in CF may be related to insulin deficiency. A better understanding of the relationship between insulin deficiency and protein breakdown in CF might lead to improved methods of treating this problem.